Great attention has been paid to ceramic materials for their heat resistance, wear resistance, high-temperature strength and other advantages. However, ceramic materials are extremely difficult to mechanically work since they are hard and brittle. Thus most ceramic articles are prepared by sintering and precursor methods. In the sintering method, a ceramic material in powder form is pressed or otherwise molded into a desired shape and then fired for sintering. The precursor method is by melting an organic polymer as a ceramic precursor or dissolving it in a suitable solvent, molding the melt or solution into a desired shape, and then firing for converting the polymer into inorganic form. The precursor method is characterized by the potential manufacture of ceramic articles to a configuration which cannot be achieved with the powder sintering method, and especially adapted for the manufacture of fibers.
Among ceramics, SiC and Si.sub.3 N.sub.4 are of great interest for high-temperature performance, more particularly because of heat resistance and high-temperature strength for the former and thermal shock resistance and fracture toughness for the latter. Extensive research works have been made on their precursors. The silicon carbide and nitride ceramic materials are also considered useful as reinforcements for fiber-reinforced composite materials by taking advantage of their light weight, heat resistance, and high strength features. Thus integration of these reinforcements with plastics, metals and ceramics is also an important subject.
The inventors previously proposed a process for manufacturing organic silazane polymers in U.S. Pat. No. 4,847,345. This method is successful in the commercial manufacture of organic silazane polymers which have a constant degree of polymerization, moldability, process ability, high strength, flexibility and ease of handling, can be effectively infusibilized and converted into ceramic material in high yields, and are thus useful as ceramic fiber precursors. Using the silazane polymers, high quality ceramic materials composed of SiC and Si.sub.3 N.sub.4 can be manufactured in high ceramic yields. There can be manufactured ceramic fibers and sheets having improved strength and mcdulus of elasticity.
Continuing investigations, however, the inventors have found that the organic silazane polymers prepared by the previously proposed method have the following problems.
(1) They are relatively unstable against heating and gradually increase their viscosity upon long.term heating during melt spinning. The melt becomes thermally less stable and less spinnable so that frequent breakage occurs during spinning.
(2) They are relatively less resistant against hydrolysis. After they are molded into articles as by spinning, the articles are liable to gradual hydrolysis by moisture in the air. Introduction of oxygen in relatively large amounts can cause ceramic articles to lose their quality.